1. Field of the Invention
This invention relates generally to manufacture of carbon filter blocks. More particularly the invention relates to formation of carbon based filter blocks by sintering a mixture of carbon particles with a polymeric binder.
2. Description of Related Art
Porous filter blocks made of carbon for filtering liquids, such as water, are widely used in industries that depend on filtration. The typical shape of such filter blocks, although others are certainly possible, is a hollow cylinder, in which the liquid enters through a center cavity and diffuses outwardly through the porous cylinder. The carbon (as well other ingredients that may be added for specificity) is distributed more or less uniformly through the cylinder by binding it to polymeric particles of various types.
The general process of manufacturing such carbon filter blocks is to prepare a thoroughly mixed powder consisting of carbon particles and the polymer or polymers of interest. The size of the particles and the specific molecular species of the polymer may vary, as well as the relative proportions, according to the required filtration performance desired. The mixture is placed into a mold by manual or automatic filling means, compacted by pressure or vibration, and then heated. The temperature of the mixture must be brought to just that temperature at which the polymer becomes “sticky”, but does not otherwise change its properties. This temperature, called in the art the “vicat” temperature, may vary according to the polymeric species. At the vicat, the carbon becomes attached to the surface of the polymer particles, and the polymer particles to each other, so that upon cooling a solid, yet porous block is formed. Precise control of the temperature is critical: too low of a temperature and the polymers will not bind the matrix; too high of a temperature, and the polymers may melt, burn, de-polymerize, or otherwise become unsuited for the desired filtration purpose. Those skilled in the art will recognize that the mixture constitutes a material with a very low conductivity for heat; it is in fact more nearly an insulator. Therefore, the prior art heating and cooling process will be slow, and also difficult to control precisely if done by external heating of the mold.
The most common method of manufacture for sintered carbon block is oven heating the base materials in a mold. The advantage of this method is relative simplicity; the disadvantage is the long process cycle time. This method uses individual steel molds, heated in a furnace to the vicat temperature, then removed and cooled to a point where the block can be removed from the mold. The heating process is slow, typically 30–60 minutes, with a similar cool down time. This time is controlled by the heat transfer rate through the mold structure and then through the carbon-binder mixture. Increasing the furnace temperature to increase the heat transmission rate is limited by the requirement that the binder particles not be damaged, as previously discussed.
A second common prior art method is to use a mold that includes a surrounding jacket containing a heated fluid or gas such as oil, steam, a fluorocarbon, or other hot gases. This method is also limited by the slow rate of heat transfer through the jacket wall into the mold, and then through the poorly conducting mixture. As in the oven method, heating is by conductive heat transfer through a low conductivity material. Process cycle times are about the same as in the oven method, although temperature limits can be controlled more precisely, especially if saturated steam is used. Introducing heat from a hot fluid from all sides of the mold simultaneously offers some improvement in process time.
Another method involves electrical heating. There are at least two approaches using electrical heating. The first simply involves wrapping the mold with a resistive electrical heating coil. As in the oven and the fluid jacket methods, this approach is ultimately limited by the heat transmission times. In addition, there can be some non-uniformity of heating patterned after the geometry of the coils. The second electrical method uses direct heating, whereby a current is applied directly to the mixture via suitable electrodes, and the mixture is heated very rapidly by resistive heating. This method is limited to those mixtures that are electrically conductive, e.g., having a high percentage of carbon. While potentially rapid, process temperature control can be only approximate, and in fact will usually be too high. In addition, short-circuiting through areas of slightly higher conductive particle content can cause internal arcing and possibly ignition of the materials.
Two other less common prior art methods can be mentioned. The first involves the use of inductive heating, whereby energy is transferred from large, suitably designed induction coils surrounding the mold. The other is microwave heating. Inductive heating can be very rapid, but doesn't work well with nominally non-conducting materials. It is also quite expensive to design, construct and maintain. Even so, temperature control and uniformity is a problem. Microwave heating requires that the target material have high hydrogenous content. While the binders are typically hydrocarbons, their response to microwaves varies widely, and in particular are subject to temperature non-uniformity, just as are food items in the home microwave.
There is therefore one object of the invention to provide a simple yet effective method for sintering carbon-binder filter blocks that improves manufacturing time and efficiency while precisely controlling the process temperatures as required.